The invention relates to a process for laser-plasma hybrid welding.
U.S. Pat. No. 6,034,343 describes a fixture in which a laser welding process and a conventional plasma welding process, for example tungsten inert gas welding (TIG) or metal active gas welding (MAG), are combined with one another. In this case, an arc burns between a non-melting electrode, usually a tungsten electrode, and the work piece, with the work piece being partially melted. The laser beam is focused onto the work piece by means of a lens system. The lens system and the electrode are arranged concentrically with respect to one another. The laser beam can then be used to concentrate a high energy output in a narrow and deep area of the partially melted work piece.
A further process for the welding of work pieces by means of laser-plasma hybrid welding is described in German publication DE 195 00 512 A1. In this case, the laser beam used to weld the work pieces and the arc are arranged at an angle to one another. In this known process too, the arc burns between an electrode and the work piece.
The known processes have proven to have the drawback that the welding speed is relatively low on account of the current intensity being limited by the service life of the electrode, leading to a limited arc power. Moreover, on account of the relatively high thermal conductivity of the work pieces which are to be welded, a considerable proportion of the heat which is introduced into the work piece flows away into the environment of the weld seam. This results in further drawbacks with regard to a high thermal load on the work piece, leading to considerable distortion of the work piece.
A further drawback is the restricted use of process gases. For example, it is customary for noble gases to be used as process gas in the known processes. The use of oxygen, for example, and other process gases which are corrosive with respect to the electrode materials is not possible.
On account of the high arc divergence of the plasma jet and the associated low power density introduced into the work piece, a high-power laser is required for the known laser-plasma hybrid welding processes.
Further drawbacks include the low long-term stability and also the cost-intensive structure and operation of conventional laser-plasma hybrid welding systems.
It is an object of the invention to provide a process for laser-plasma hybrid welding in which the investment and operating costs of the laser-plasma hybrid welding process can be reduced and the welding speed can be increased.
This object is achieved by the claimed invention. Advantageous embodiments of the invention form the subject matter of subclaims.
According to the invention, a free microwave-induced plasma jet is used for the laser-plasma hybrid welding, and this jet is generated in the following way: microwaves which are guided in a hollow waveguide are generated in a high-frequency microwave source. The process gas is introduced into a microwave-transparent tube, which comprises a gas inlet opening and a gas outlet opening, at a pressure p≧1 bar, through the gas inlet opening of the tube, in such a manner that it has a tangential flow component. A plasma is generated in the microwave-transparent tube by means of electrode-free ignition of the process gas, and this plasma is then introduced into the working space through a metallic nozzle arranged at the gas outlet opening of the tube, with the result that the plasma jet is generated. The work piece to be welded is in particular located in the working space.
The laser beam is advantageously generated in a solid-state laser, in particular an Nd-YAG laser, or in a gas laser, in particular a CO2 laser or excimer laser. However, it is also possible for the laser beam to be generated in a diode laser.
In a first advantageous embodiment of the invention, the laser beam runs through the microwave-transparent tube and through the opening of the nozzle into the working space. In this case it is possible; for example, for the laser beam and the plasma jet to run concentrically with respect to one another. Given a correspondingly large opening diameter of the nozzle, however, it is also possible for the laser beam and the plasma jet to run at a predeterminable angle, which is limited by the geometry of the arrangement, with respect to one another. The advantage of this embodiment is that the laser beam plays a supporting role in the ignition and maintenance of the plasma. Moreover, this makes it possible to realize a compact structure of a laser-plasma hybrid welding process.
In a second advantageous embodiment of the laser-plasma hybrid welding process according to the invention, the laser beam runs outside the microwave-transparent tube. In this case, it is possible, in a suitable arrangement, for the laser for generating the laser beam outside the hollow waveguide arrangement for generation of the plasma jet to be positioned in such a manner that the laser beam and the plasma jet cross one another on the surface or in the region close to the surface of the work piece which is to be welded. Furthermore, the laser beam and the plasma jet may also be arranged in such a manner with respect to one another that the laser beam precedes the plasma jet in the welding process, or vice versa. This allows the quality of the weld seam to be improved and the welding speed to be increased.
Particularly advantageous plasma properties result by means of the electrode-free laser-plasma hybrid welding process according to the invention. For example, the specific enthalpy of the plasma and the associated enthalpy flow density of the plasma are increased. In view of this effect, the plasma temperature of the plasma and of the plasma jet is increased. This provides advantages in terms of an increased welding speed and lower weld seam costs compared to the laser-plasma hybrid welding process disclosed in the prior art. The laser-plasma hybrid welding process according to the invention therefore provides an electrode-free laser-plasma hybrid welding process which offers considerable advantages in terms of operating economics and applications combined, at the same time, with a wide range of uses for the welding process.
Moreover, the properties of the plasma jet with regard to a reduced diameter and a reduced jet angle divergence are improved. Furthermore, the cylinder-symmetrical plasma jet opens out in parallel in the process according to the invention.
The tangential feeding of the process gas into the microwave-transparent tube causes an axial flow component directed toward the gas outlet opening of the tube to form in the tube. This assists with the generation in accordance with the invention of a plasma jet with a low beam angle divergence. On account of the radial acceleration which is caused by the process gas being fed in tangentially and is further boosted by the narrowing of the cross section of the nozzle in the direction of the nozzle outlet, the nonuniformly accelerated free charge carriers move toward the nozzle outlet on ever narrower spiral paths, with the result that the centripetal acceleration of the charge carriers increases. This movement is maintained by the charge carriers even after they have emerged from the nozzle into the working space. Since there is no local charge neutrality on account of the different ion and electron mobility, an axially oriented magnetic field, which leads to a flow constriction of the plasma jet after it emerges from the nozzle (z pinch), is induced in the plasma jet. This is because of the magneto-hydrodynamic effect (MHD effect).
A further advantage of the process according to the invention is that the plasma jet can be generated by means of inexpensive and robust high-frequency systems, e.g. a magnetron or klystron. These high-frequency systems advantageously make microwave sources in the required power range up to 100 kW and frequency range from 0.95 GHz to 35 GHz available. In particular, it is possible to use microwaves with a frequency of 2.46 GHz, since this involves inexpensive microwave sources which are in widespread use in industry and domestic applications.
On account of the electrode-free generation of plasma, there is no restriction in the process according to the invention in terms of the process gases which can be used. Therefore, the process according to the invention solves the problem of the prior art whereby in the case of electron-induced plasmas reactions occur between the process gases used and the electrode materials, for example leading to the formation of tungsten oxide or tungsten nitride in the case of tungsten electrodes or leading to hydrogen embrittlement. It is therefore possible for the specific enthalpy of the plasma to be increased, in combination with an improved heat conduction between plasma and work piece, by suitable selection of gases or gas mixtures which are appropriate to the process.
Moreover, on account of the electrode-free laser-plasma hybrid welding, the introduction of undesired electrode material into the weld metal is avoided. Furthermore, a disruption-free, unmanned and automated welding process is possible without worn parts having to be constantly replaced.
A further advantage of the laser-plasma hybrid welding process according to the invention is that the heat-affected zone of the plasma jet on the work piece is significantly reduced, which leads to a lower introduction of heat, a reduced work piece distortion and a drop in the levels of damage to the material. Moreover, the laser-plasma hybrid welding process according to the invention allows defect-free welding in terms of reduced weld undercuts and a low porosity in the weld seam.
To ensure reliable operation and reliable ignition of the plasmas required for the process according to the invention, in an advantageous embodiment of the invention the cross section of the hollow waveguide which is provided for guiding the microwaves is narrowed. In this case, the hollow waveguide is preferably narrowed at the location at which the microwave-transparent tube is guided through the hollow waveguide. The hollow waveguide and the tube are oriented perpendicular to one another in an expedient embodiment of the invention. The advantage of this is an increase in the electrical field strength at the location of the narrowed cross section. This firstly improves the ignition properties of the process gas and secondly increases the power density of the plasma.
In a further advantageous embodiment of the invention, it is also possible for a spark gap to be used to ignite the plasma.